Convert all camel cased variables and options to snake_case

This may seem ridiculous/unnecessary but the more I've worked with data, the more I've preferred snake_case. It adds a predictability and clarity that avoids a lot of errors. Not only that, but it works well with dot notation in JavaScript.

Mild pain in the butt, but it will be worth it from a maintenance perspective.

For any existing API options that leverage camelCase (e.g., accounts APIs with emailAddress), leave that as an option but prefer email_address in the docs and support it internally.

Done. Where appropriate, the legacy camelCase is still supported.